Many electric devices at present are driven by a specific potential. Hence power conversion circuits are widely used to meet this purpose. Improvements are constantly made to achieve more stable output voltage and higher conversion efficiency. Power conversion is affected by many factors, such as loss of elements and loss caused by leakage or mutual offset when energy is transformed to magnetic force during power conversion. As a great amount of current passes through the transformer in the conversion circuit, leakage inductance stored in the transformer constantly stores energy and generates a reverse surge (shown by the front edge of square waveforms in FIG. 1) while the power switch in the conversion circuit is switched repeatedly. The reverse surge constantly applies to the elements connected to the transformer, as a result the performance of the peripheral elements (such as power elements, capacitors and the like) deteriorates after a period of time, even the life span of the elements could suffer. Moreover, if the reverse surge energy is not properly guided, power conversion efficiency drops. Even if the reverse surge energy has been properly guided, it still exists in the loss, hence total conversion efficiency of the circuit does not improve significantly.
In order to improve the conversion efficiency to meet increasingly enhanced energy regulations of various countries, many techniques have been provided in prior art to address the aforesaid surge problem. For instance, R.O.C. patent No. 583830 entitled “Converter equipped with an active LC snubber circuit” discloses an LC snubber circuit to suppress the surge and recover the surge energy for reusing. The LC snubber circuit includes a diode, a capacitor, a second transformer and a second switch. Its circuit contains a main transformer coupled with a clamp capacitor (Cs) to store energy. Through the diode the reverse surge energy is transmitted from the clamp capacitor (Cs) to the second transformer. The secondary side of the second transformer and the second switch and aforesaid capacitor recover the reverse surge energy to supply to a second load. It can suppress the surge and recover the energy, but still has a drawback, namely requires additional second transformer and second switch. Not only its cost is higher, more space has to be allocated and occupied in the limited interior space of the power circuit casing. Moreover, although the reverse surge energy is recovered to supply to the load, it has to go through transformation via the second transformer, and loss occurs by one more time of energy transformation to the secondary side. The additional cost outweighs the generated benefit.